Communication systems typically include a plurality of communication devices, such as mobile or portable radio units, dispatch consoles and base stations, which are geographically distributed among various base sites and console sites. The radio units wirelessly communicate with the base stations and each other using radio frequency (RF) communication resources, and are often logically divided into various subgroups or talk-groups. The base stations are hard-wired to a controller that controls communications within the system.
In a time division multiple access (TDMA) system, for example, voice transmission channels are divided into periodically repeated superframes, each of which includes multiple digitized voice bursts. Typically, the first burst in each superframe includes a voice frame synchronization pattern surrounded by encoded voice information. The remaining bursts may include link control information in the center of the encoded voice information instead of the voice frame synchronization pattern.
In such TDMA systems, a typical method for ending a voice call is for the transmitting radio unit to send a stand-alone termination burst following the last burst of the superframe during which the end of call event is detected. The termination burst generally contains a data synchronization pattern that is a symbol complement to the voice frame synchronization pattern, thus minimizing the risk of mistakenly terminating a call.
This method of terminating a voice call, however, has several drawbacks. First, when a dekey event indicates the end of the voice call before the last burst in the superframe, the radio unit must nonetheless keep transmitting the remaining bursts with some predetermined information, as the termination burst can only be transmitted after the last burst in the superframe. As a result, the slot channel remains occupied (i.e., the call is still technically “active”) until the end of the superframe even though the dekey event occurred earlier in the superframe, which prevents other units from using the slot channel during that time.
Additionally, with some call scenarios, such as on takeovers with a console call interrupting a voice call, audio from the interrupting source must be buffered until the current call has properly been terminated at the end of a superframe so that the interrupting audio can be sent over the air. These interruptions may happen multiple times during a single call. Each time this happens, a delay up to the duration of the superframe may be introduced with the baseline operation. This delay will remain present until the call ends.
Accordingly, there is a need for a system and method of terminating a voice call in any burst within a multi-burst superframe in a more efficient manner than the method described above.
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